From e33aaa0a453d08fa29f18ee6fbf5dd7a31a0c7b6 Mon Sep 17 00:00:00 2001
From: Jamie Portsmouth <jamie.portsmouth@autodesk.com>
Date: Sat, 1 Jun 2024 00:48:36 +0100
Subject: [PATCH] Implement TIR fix in MaterialX (#203)

---
 reference/open_pbr_surface.mtlx | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/reference/open_pbr_surface.mtlx b/reference/open_pbr_surface.mtlx
index e9462bb..a72e3f3 100644
--- a/reference/open_pbr_surface.mtlx
+++ b/reference/open_pbr_surface.mtlx
@@ -304,12 +304,23 @@
     </multiply>
 
     <!-- Dielectric Base -->
-    <divide name="specular_to_coat_ior_ratio" type="float">
-      <input name="in1" type="float" interfacename="specular_ior" />
-      <input name="in2" type="float" interfacename="coat_ior" />
-    </divide>
+      <!-- apply IOR ratio inversion method to avoid TIR artifact (as in Coat TIR section of spec) -->
+      <divide name="specular_to_coat_ior_ratio" type="float">
+         <input name="in1" type="float" interfacename="specular_ior" />
+         <input name="in2" type="float" interfacename="coat_ior" />
+      </divide>
+      <divide name="coat_to_specular_ior_ratio" type="float">
+         <input name="in1" type="float" interfacename="coat_ior" />
+         <input name="in2" type="float" interfacename="specular_ior" />
+      </divide>
+      <ifgreater name="specular_to_coat_ior_ratio_tir_fix" type="float">
+         <input name="value1" type="float" nodename="specular_to_coat_ior_ratio" />
+         <input name="value2" type="float" value="1.0" />
+         <input name="in1" type="float" nodename="specular_to_coat_ior_ratio" />
+         <input name="in2" type="float" nodename="coat_to_specular_ior_ratio" />
+      </ifgreater>
     <mix name="eta_s" type="float">
-      <input name="fg" type="float" nodename="specular_to_coat_ior_ratio" />
+      <input name="fg" type="float" nodename="specular_to_coat_ior_ratio_tir_fix" />
       <input name="bg" type="float" interfacename="specular_ior" />
       <input name="mix" type="float" interfacename="coat_weight" />
     </mix>